Light-scattering spectroscopy entails illumination of a substance and analyzing light that is scattered at angles relative to the incident source. The photon-matter interactions of the scattering events may be either elastic or inelastic. In an inelastic event, a photon's energy (wavelength) changes as a result of the light-matter interaction. In an elastic event, a photon's energy (wavelength) does not change. Absorption, the phenomena in which a fraction of photons are entirely absorbed, also plays a role in light-scattering spectroscopies.
Raman, diffuse reflectance, and fluorescence spectroscopies are of particular interest as they relate to vibrational and nonvibrational photonic responses of a material. The Raman effect describes a subtle light-matter interaction. Minute fractions of light illuminating a substance are Raman-scattered in random directions. Raman-scattered light is color shifted from the incident beam (usually a laser). The color frequency shifts are highly specific as they relate to molecular bond vibrations inducing molecular polarizability changes. Raman spectroscopy is a powerful technique for chemical analysis and monitoring. Analysis of the resulting low light levels requires sophisticated, expensive instrumentation and technical complexity. The collection of Raman spectra in the fingerprint (FP) region, i.e., approximately 200 to 2,000 cm−1, through optical fibers is complicated by Raman signal from the fibers themselves. A band-pass (laser line) filter may be used at the delivery end of a delivery fiber to remove the silica Raman bands arising from the fiber itself before illuminating a sample. A long-pass filter may be disposed before a collection fiber so that only the Stokes scattered light enters the fiber. Filtering for optical fiber-based Raman spectroscopy is described, for example in U.S. Statutory Invention Registration No. H002002.
Specular reflectance relates to a surface's mirror-like aspects. Diffuse reflectance relates to light that is elastically scattered from the surface of a material at diffuse angles relative to the incident beam. For example, a projector screen diffusely reflects light while a glossy, newly waxed car has a high specular component. Diffuse reflectance spectroscopy is important for chemical analysis as well as measuring visual perception.
Fluorescence relates to substances which absorb light at one wavelength then re-emit it at a longer wavelength as a result of electronic transitions. As an example, a “highlighter” felt-tip marker appears to “glow” green as it absorbs blue and ultraviolet light then emits it as green. Fluorescence provides a powerful technique for chemical monitoring.
Raman spectroscopy involves energizing a sample with a high-power, narrow-wavelength energy source, such as a laser. The laser photons induce low intensity light emissions as wavelengths shift. The Raman effect is an inelastic scattering of photons. The emitted Raman light is collected and analyzed using a spectrometer or light detector. The spectral positions (colors) of the shifts provide fingerprints of the chemicals in the sample. Thus, Raman spectroscopy provides a means for chemical identification. The intensity of the shift (the spectral peak height) correlates to chemical concentration. Thus, a properly calibrated instrument provides chemical content and concentration. In practicality, Raman spectroscopy is technically complex and requires sophisticated, expensive instrumentation. Raman spectroscopy-based methods and apparatuses are disclosed, for example, in U.S. Pat. Nos. 5,293,872; 6,208,887 and 6,690,966, and in U.S. Publication No. 2004/0073120.
Laser-induced breakdown spectroscopy (LIBS) is another optical analytical method that may be employed using fiber optics. LIBS is disclosed in U.S. Pat. Nos. 5,751,416; 6,762,835; and 7,394,537.
Optical coherence tomography (OCT) is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue). Optical coherence tomography is an interferometric technique, typically employing near-infrared light. The use of relatively long wavelength light allows it to penetrate into the scattering medium. Confocal microscopy, another similar technique, typically penetrates less deeply into the sample. U.S. Publication No. 2004/0267110 discloses an OCT system and its use.
Dispersive spectroscopic techniques typically utilize a slit at the entrance of the spectrometer to control the input of light which affects the resolution of the spectra. Wider slits result in a lower resolution of the spectra. The larger angle of input light (higher NA) the lower the resolution of the spectra. The lower the resolution of the spectra results in more difficulty in resolving individual peaks. The narrower the slit is, the higher resolution obtained, but at the detriment of total light input to the spectrometer. This will result in longer acquisition times unless the input (fiber diameter) is the same as the slit width.
Since fingerprint region Raman fiber optic probes with high spatial resolution or controllable spatial resolution has historically been difficult to manufacture, this has been an area of concentration for the design of the present invention. Many other spectroscopic techniques including high wavenumber Raman will also work with the probe design of the present invention. U.S. Publication No. 2006/0139633 discloses the use of high wavenumber Raman spectroscopy for the characterization of tissue. Santos et al., Fiber-Optic Probes for In Vivo Raman Spectroscopy in the High-Wavenumber Region, Anal. Chem. 2005, 77, 6747-6752 discloses known probe designs for high wavenumber Raman spectroscopy.
To the inventors' knowledge, to this point no optical fiber probe has achieved the combination of very small size, robustness, ease of construction, high spatial resolution, and high collection efficiency. In addition, the ability to collect both polarization states, fluorescence, diffuse reflectance and OCT all from the same probe looking at the same spot to collect the spectra has not before been realized. The probes of the present invention meet the above criteria utilizing GRIN lenses and other optical elements and fiber optics, making them well adapted for industrial, scientific and medical applications.